This invention relates to a discharge tube arrangement and in particular, though not exclusively, to such an arrangement for use as a light source. In particular, this invention relates to a structure, known as a launcher, for such a discharge tube arrangement.
It is known, e.g. as disclosed in U.S. Pat. No. 4,049,940 (Moisan et al), to generate and sustain a discharge in a gas by using electromagnetic surface waves. Surface waves are created by a launcher which is positioned around and external of, but not extending the whole length of, a discharge tube containing the gas. In such an arrangement, it is not necessary to provide electrodes inside the discharge tube. The power to generate the electromagnetic wave is provided by a radio frequency (r.f.) power generator.
For efficient power transfer from the power generator to the discharge tube, it is necessary to match the impedance of the power supply, the launcher and the discharge tube. M. Moisan and Z. Zakrzewski "New surface wave launchers for sustaining plasma columns at submicrowave frequencies (1-300 MHz)" Rev. Sci. Instrum 58 (10), October 1987, disclose a launcher with an impedance-matching network to provide what is termed `external matching` (as opposed to `internal matching` which would be provided by the size and shape of the launcher.) A typical launcher for use with an impedance-matching network is shown in FIG. 1. The launcher 2 comprises an inner aluminium tube 4 and an outer aluminium tube 6 coaxial with the inner tube 4. One end of the outer tube 6 is closed by a steel plate 8. The inner tube 4 is shorter than the outer tube 6 and accordingly an annular launching gap 10 is defined between the end of the inner tube 4 and the steel plate 8. At the other end of the launcher, an aluminium metal plate 12 extends perpendicularly from the inner tube 4 towards the outer tube 6 almost closing that end of the launcher. An annular field arresting gap 14 between the outer edge of the plate 12 and the outer tube 6 confines the field existing between the inner and outer tubes 4, 6. This gap allows a non-zero potential difference to be generated in the launching gap 10. A Telfon disc 15 adjacent the field arresting gap 14 holds the plate 12 and the inner tube 4 in position relative to the outer tube 6 and reduces, to a certain extent, the leakage of r.f. power from the field arresting gap 14. R.f. power is supplied to the launcher via a connector 16 and an impedance matching network (not shown) consisting of inductors and capacitors. The combination of the r.f. power generator, the impedance matching network and the launcher constitute an excitation device for the gas fill in the discharge tube.
A major problem with a discharge body arrangement incorporating such a launcher is the leakage of r.f. power, producing r.f. interference, from the field arresting gap 14. Moisan et al (ibid) teach that the field arresting gap must be small to minimise field leakage outside, but not so small as to allow r.f. arcing. The r.f. interference produced by the aforementioned launcher is significant--too great for a discharge tube arrangement intended, inter alia, for use as a domestic light source.
It is an object of the present invention to provide a launcher which at least alleviates the problem outlined hereinbefore.